In the technical field of wireless communication apparatuses, such as cell phones, a demand for downsizing of an RF circuit has been increased, for example, corresponding to an increase in number of parts mounted on each apparatus with the view of realizing a higher level of performance. To meet such a demand, a further miniaturization of various parts of the circuit has been progressed by utilizing the MEMS (micro-electromechanical systems) techniques.
An MEMS switch is generally known as one of those parts. The MEMS switch is a switching device in which various components are formed in very small sizes by the MEMS techniques, and it includes at least one pair of contacts which are mechanically opened and closed to perform switching, a driving mechanism for achieving the mechanical opening and closing operations of the contact pair, and so on. When the MEMS switch is applied to the switching of a high-frequency signal on the GHz order, in particular, the MEMS switch tends to exhibit a higher degree of isolation in the open state and a lower insertion loss in the closed state than other switching devices using, e.g., PIN diodes and MESFETs. Such a tendency is attributable to the facts that the open state is established by spacing mechanically formed between the contact pair, and that parasitic capacitance is small because the MEMS switch is a mechanical switch. Known MEMS switches are described in, e.g., Japanese Unexamined Patent Application Publication No. 2004-1186 and No. 2004-311394, and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-528751.
FIGS. 51 to 53 represent a switching device Z1 as one example of the typical switching devices. Specifically, FIG. 51 is a plan view of the switching device Z1. FIG. 52 is a plan view, partly omitted, of the switching device Z1. FIG. 53 is a sectional view taken along a line LIII-LIII in FIG. 51.
The switching device Z1 includes a substrate S3, a signal line 91, a driving line 92, and a movable line 93 (omitted in FIG. 52). The signal line 91 is formed by patterning on the substrate S3. As illustrated in FIG. 53, the signal line 91 has a contact portion 91a capable of contacting the movable line 93. The driving line 92 is formed by patterning on the substrate S3, and it has a driving electrode portion 92a. The movable line 93 is formed in a shape protruding upwards from the substrate S3, as illustrated in FIG. 53, by a plating process, for example. The movable line 93 includes a projected portion or a contact portion 93a, which is capable of contacting the signal line 91, and a portion positioned to face the driving electrode portion 92a of the driving line 92. The signal line 91, the driving line 92, and the movable line 93 are each made of a predetermined conductive material.
In the switching device Z1 having the above-described structure, when a predetermined driving voltage is applied to the movable line 93 in a state where the driving line 92 is connected to the ground, an electrostatic attraction force is generated between the driving electrode portion 92a of the driving line 92 and the movable line 93, whereby the movable line 93 is partly operated or elastically deformed until the contact portion 93a of the movable line 93 comes into contact with the contact portion 91a of the signal line 91. The closed state of the switching device Z1 is thus established. In the closed state, the signal line 91 and the movable line 93 are connected to each other so that a current is allowed to pass between the signal line 91 and the movable line 93. With such a switching-on operation, the on-state of a high-frequency signal can be achieved.
On the other hand, when, in the switching device Z1 in the closed state, the application of the voltage to the movable line 93 is stopped to extinguish the electrostatic attraction force acting between the driving electrode portion 92a and the movable line 93, the movable line 93 returns to its natural state and the contact portion 93a of the movable line 93 moves away from the contact portion 91a of the signal line 91. The open state of the switching device Z1 is thus established. In the open state, the signal line 91 and the movable line 93 are electrically separated from each other, whereby a current is prevented from passing between the signal line 91 and the movable line 93. With such a switching-off operation, the off-state of a high-frequency signal can be achieved. Further, the switching device Z1 in the open state can be changed again to the closed state, i.e., the on-state, with the switching-on operation described above.
In the switching device Z1, the movable line 93 serves as, together with the signal line 91, a passage route for the high-frequency signal, and the driving voltage is applied to the movable line 93 having the portion that is positioned to face the driving electrode portion 92a of the driving line 92 (namely, the movable line 93 serves as not only a signal line, but also a driving line). Because the parasitic capacitance between the movable line 93 and the driving electrode portion 92a positioned to face the movable line 93 is comparatively large, the high-frequency signal that is to pass through the movable line 93 is apt to leak to the driving line 92 through a region where the driving electrode portion 92a and the movable line 93 are positioned to face each other. In other words, an insertion loss is apt to generate in the switching device n. As the frequency of the signal becomes higher, an extent of signal leakage to the driving line 92 increases and the insertion loss also tends to increase. In that type of the switching device Z1, a superior high-frequency characteristic is hard to obtain.
FIGS. 54 to 56B illustrate a switching device Z2 as another example of the known switching devices. FIG. 54 is a plan view of the switching device Z2. FIG. 55 is a plan view, partly omitted, of the switching device Z2. FIGS. 56A and 56B are sectional views taken along a line LVIA-LVIA and a line LVIB-LVIB in FIG. 54, respectively.
The switching device Z2 includes a substrate S4, a stationary portion 94, a movable portion 95, a signal line 96A, a pair of signal lines 96B (omitted in FIG. 55), a driving line 97A, and a driving line 97B (omitted in FIG. 55). As illustrated in FIGS. 56A and 56B, the stationary portion 94 is joined to the substrate S4 through a boundary layer 98. As most clearly illustrated in FIG. 55, the movable portion 95 includes a fixed end 95a fixed to the stationary portion 94, and a free end 95b, and it is surrounded by the stationary portion 94 with a slit 99 interposed there between. The stationary portion 94 and the movable portion 95 are integrally formed on a single silicon substrate. As most clearly illustrated in FIG. 55, the signal line 96A is disposed on the movable portion 95 near the free end 95b thereof and has contact portions 96a capable of contacting the signal lines 96B, respectively. The signal lines 96B are each formed in a shape protruding upwards from the stationary portion 94, as illustrated in FIG. 56A, by a plating process, for example. Further, each of the signal lines 96B has a projected portion or a contact portion 96b, which is capable of contacting the signal line 96A. As most clearly illustrated in FIG. 55, the driving line 97A is disposed to extend over the stationary portion 94 and the movable portion 95 and has a driving electrode portion 97a on the movable portion 95. The driving line 97B is formed in a shape protruding upwards from the stationary portion 94, as illustrated in FIG. 56B, by a plating process, for example, and has a portion positioned to face the driving electrode portion 97a of the driving line 97A. The signal lines 96A and 96B and the driving lines 97A and 97B are each made of a predetermined conductive material.
In the switching device Z2 having the above-described structure, when a predetermined driving voltage is applied to the driving line 97A in a state where the driving line 97B is connected to the ground, an electrostatic attraction force is generated between the driving electrode portion 97a of the driving line 97A and the driving line 97B. When the electrostatic attraction force is sufficiently large, the movable portion 95 is operated or elastically deformed until the contact portions 96a of the signal line 96A come into contact with the contact portions 96b of the signal lines 96B. The closed state of the switching device Z2 is thus established. In the closed state, the pair of signal lines 96B are electrically bridged there between through signal line 96A so that a current is allowed to pass between the pair of signal lines 96B. With such a switching-on operation, the on-state of a high-frequency signal can be achieved.
On the other hand, when, in the switching device Z2 in the closed state, the application of the voltage to the driving line 97A is stopped to extinguish the electrostatic attraction force acting between the driving electrode portion 97a and the driving line 97B, the movable portion 95 returns to its natural state and the contact portions 96a of the signal line 96A on the movable portion 95 move away from the contact portions 96b of the signal lines 96B. The open state of the switching device Z2 is thus established. In the open state, the pair of signal lines 96B are electrically separated from each other, whereby a current is prevented from passing between the pair of signal line 96B. With such a switching-off operation, the off-state of a high-frequency signal can be achieved. Further, the switching device Z2 in the open state can be changed again to the closed state, i.e., the on-state, with the switching-on operation described above.
In the switching device Z2, two gaps G′ between the two pairs of contact portions 96a and 96b, illustrated in FIG. 56A, may differ from each other due to variations occurred in manufacturing operations when the switching device Z2 is not driven (i.e., when the movable portion 95 is in its natural state). In such a case, even when the predetermined voltage is applied to the driving line 97A, the movable portion 95 is not elastically deformed to such an extent that one pair of contact portions 96a and 96b, which form the larger gap G′, can be brought into the closed state, thus causing a failure that the switching device Z2 is not turned to the on-state. When the two gaps G′ illustrated in FIG. 56A differ from each other in the not-driven state, the movable portion 95 can be elastically deformed, by applying a sufficiently high voltage to the driving line 97A, such that after one pair of contact portions 96a and 96b forming the smaller gap G′ have been brought into the closed state, the other pair of contact portions 96a and 96b forming the larger gap G′ are also brought into the closed state. With such a voltage application, however, because an excessive load is eventually imposed between the contact portions 96a and 96b which have been brought into the closed state at earlier timing, a sticking failure, i.e., a phenomenon of sticking to the contact state due to application of excessive pressure, tends to occur between the contact portions 96a and 96b which have been brought into the closed state at the earlier timing. Such a tendency to cause the sticking failure is not preferable including in realizing a long contact opening/closing life.